In a liquid crystal image forming element which is used in an image projecting apparatus, a polarization component of one direction is only effectively utilized and a polarization component of the other direction orthogonal to the above direction causes degradation of contrast. Therefore, the polarization direction is controlled by disposing a polarizer at the front stage and the back stage of the liquid crystal image forming element. However, light beams emitted from a light source are non-polarization beams whose polarization directions are not the one direction. Therefore, when the polarization component of the one direction is selected by a polarizer, light quantity also becomes a half.
In order to solve this problem, a polarization conversion element, which efficiently causes the non-polarization beams emitted from the light source to be light beams in one direction, is generally disposed in the front stage of the polarizer. In the polarization conversion element, the non-polarization beams are basically separated into P polarization and S polarization by a polarization beam splitter, and one of the direction of the separated polarization is rotated by 90° by a method and the directions of both the polarization are made to be the same direction. Consequently, the directions of both the beams become equal (refer to, for example, Patent document 1).
There are generally two methods that rotate the polarization direction of one of the separated polarization. In a first method, a ½ wavelength plate is used, that is, refractive index difference between the two directions is used. In a second method, two mirrors are used, that is, the reflection directions of the two mirrors are suitably determined while the separated light beams are reflected by the two mirrors, and the directions of the total beams are made the same directions. In the second method, since the reflection by the mirrors is used, the conversion can be efficiently performed regardless of wavelengths; however, since illumination beams from the image projecting apparatus have a certain thickness, the size of the polarization conversion element which uses the mirrors becomes large and also it is not easy to form an optical system which combines the separated beams. When the polarization conversion element is divided into many pieces and the many pieces are arrayed, the polarization conversion element can be thin; however, arraying the mirrors is logically possible, but actually difficult. On the other hand, in the first method, since the ½ wavelength plate is used, it is easy to array the ½ wavelength plates. In addition, since a fly-eye lens which is disposed on the illumination optical path of the image projecting apparatus can be used with the ½ wavelength plate, the second method is generally used.
FIG. 27 is a diagram showing a transmission type liquid crystal image projecting apparatus. FIG. 28 is a diagram showing a reflection type liquid crystal image projecting apparatus. Both the types use a liquid crystal image forming element and have polarization selectivity. Further, in both types, almost the same illumination optical system is used.
First, operations of the transmission type liquid crystal image projecting apparatus shown in FIG. 27 are described. Non-polarization beams emitted from a light source 1001 such as a white lamp become approximately parallel beams at a reflector 1002 and the approximately parallel beams are input in an optical integrator 1003. The optical integrator 1003 makes illuminance of light which is irradiated on an image forming element uniform and is composed of a pair of fly-eye lenses 1003-1 and 1003-2. A fly-eye lens is a lens array in which lenses are arrayed in the length and width directions. Each lens in the fly-eye lens has an analogous shape with an image forming element.
A light beam transmitted through each lens in the first fly-eye lens 1003-1 is projected on an image forming element by the second fly-eye lens 1003-2 and a condenser lens 1005 disposed behind the second fly-eye lens 1003-2. With this, the illuminance distribution on the image forming element is made uniform.
Light beams output from the optical integrator 1003 are input in a polarization conversion element 1004. In the polarization conversion element 1004, polarization beam splitters, reflection film surfaces, ½ wavelength plates, and so on are arrayed corresponding to the pitch of the fly-eye lens. The polarization conversion element 1004 efficiently converts the non-polarization beams into light beams in one polarization direction. The light beams output from the polarization conversion element 1004 are transmitted through the condenser lens 1005 and are reflected by a reflection mirror 1006, and the reflected light beams are separated into red light beams, green light beams, and blue light beams by dichroic mirrors 1007 and 1008. The separated light beams are irradiated on the image forming elements.
For example, light beams transmitted through the first dichroic mirror 1007 are reflected by a mirror 1009 and the reflected light beams are irradiated on a liquid crystal element for red 1015-1 by being transmitted through a condenser lens for red 1014-1. Light beams reflected by the first dichroic mirror 1007 are reflected by a second dichroic mirror 1008 and the reflected light beams are irradiated on a liquid crystal element for green 1015-2 by being transmitted through a condenser lens for green 1014-2. Light beams transmitted through the second dichroic mirror 1008 are irradiated on a liquid crystal element for blue 1015-3 by being transmitted through a condenser lens for blue 1014-3 via a lens 1010, a mirror 1011, a lens 1012, and a mirror 1013.
The liquid crystal elements 1015-1, 1015-2, and 1015-3 are image forming elements and modulate the corresponding light beams based on image signals of red, green, and blue components. The light beams transmitted through the liquid crystal elements 1015-1, 1015-2, and 1015-3 are made composite light beams by a color composite prism 1016 and the composite light beams are projected on a screen 1018 by a projection lens 1017.
Operations of the reflection type liquid crystal image projecting apparatus shown in FIG. 28 are basically the same as those described in FIG. 27. However, the polarization light beams reflected by the reflection mirror 1006 are separated into red, green, and blue polarization light beams by an optical system composed of a dichroic prism 1021, mirrors 1022 and 1023, and a dichroic mirror 1024. The red polarization light beams are transmitted through a polarization beam splitter for red 1025-1 and a reflection type liquid crystal for red 1026-1, the green polarization light beams are transmitted through a polarization beam splitter for green 1025-2 and a reflection type liquid crystal for green 1026-2, and the blue polarization light beams are transmitted through a polarization beam splitter for blue 1025-3 and a reflection type liquid crystal for blue 1026-3. The transmitted through polarization light beams are made composite light beams by the color composite prism 1016. The above operations are different from those in FIG. 27.
FIG. 29A is a diagram showing a polarization conversion element which is used in a liquid crystal image projecting apparatus. FIG. 29B is a diagram showing another polarization conversion element which is used in a liquid crystal image projecting apparatus.
In the polarization conversion element shown in FIG. 29A, a unit 2020 is composed of a polarization separation film 2021, a reflection film 2022, and a ½ wavelength plate 2023. The units 2020 are arrayed to accommodate the pitch of a fly-eye lens 2000. The polarization separation film 2021 has the gradient of 45° for the input light axis and the reflection film 2022 is disposed parallel to the polarization separation film 2021. Each light beam 2010 output from the fly-eye lens 2000 is input in each unit 2020 and is separated into transmission light (P polarization) and reflection light (S polarization) by the polarization separation film 2021, and the reflection light is further reflected by the reflection film 2022 and becomes a light beam parallel to the transmission light. The polarization direction of either of the transmission light (P polarization) or the reflection light (S polarization) is rotated by the ½ wavelength plate 2023 and the polarization directions become equal. In this case, the reflection light (S polarization) is rotated. With this, the light beams 2010 which are non-polarization at the input time are converted into light beams having the same polarization direction. In FIG. 29B, the reflection film 2022 shown in FIG. 29A is replaced by a polarization separation film 2024 similar to the polarization separation film 2021, and operations are almost the same as those in FIG. 29A.
[Patent Document 1] Japanese Patent No. 3492355
In the polarization conversion element shown in FIGS. 29A and 29B, a non-polarization beam can be efficiently converted into a light beam having one polarization direction. However, in the polarization conversion element shown in FIG. 29A, two surfaces are needed, that is, the polarization separation surface and the reflection surface are required for one pitch of the fly-eye lens. In other words, two blocks are needed, that is, a block in which the polarization separation surface is formed and a block in which the reflection surface is formed are required. Consequently, twice as many blocks as the number of lenses in the fly-eye lens must be bonded. Therefore, many man-hours are required. In the actual mass-production, a glass plate on which the polarization separation film is formed and a glass plate on which the reflection film is formed are alternately bonded and the bonded product is cut in the direction of 45° for the bonded surface and polished. That is, two kinds of glass plates are prepared and twice (+both ends) as many bonding times as the number of the lenses in the fly-eye lens are needed.
Therefore, as described above, the many man hours are required. As shown in FIG. 29B, the polarization separation film can also work as the reflection film; however, two films (surfaces) are also needed for one lens in the fly-eye lens, and the number of bonding times of glass plates cannot be reduced.